The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes and filter cartridges. For convenience herein, the term “filter element” will be used to refer collectively to these types of filtration means.
Filter elements are typically constructed from felts and/or fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, glasses, cellulose and fluoropolymers. Selection of the type of material used is typically based on the gas stream with which the filter element comes in contact, the operating conditions of the system, the type of particulate being filtered and cost.
Depth filtration techniques utilize the filter element to stop particles through the depth of the element. As the particles build up on the element, the filtration efficiency of the element is increased. After an amount of dust has caked on the filter element, the flow rate of gas through the element is reduced to a level where the filter must be replaced or the bulk dust cake removed from the surface of the element. Typically the dust cake is removed by some form of agitation, such as a pulse of compressed air, vibration, shaking or the like.
Nonwoven cellulose paper filters comprise loosely assembled webs or masses of fibers bound together with an adhesive binder. Adequately bonded cellulose paper filters have advantages over other non-woven media and woven fabrics for a large variety of uses. A significant advantage of non-woven cellulose paper media is lower cost. The cost effectiveness makes cellulose paper filter media particularly attractive in consumer products, such as filters for household vacuums. Many such vacuums use low cost filter media comprising cellulose paper. The nonwoven cellulose paper filter may be in the form of a pleated cartridge or a bag.
It is known to form cellulose paper filters by impregnating, printing or otherwise depositing an adhesive bonding composition on a base web of cellulose fibers. The base web of cellulose fibers to which the binder is applied can be produced by carding, garnetting, air-laying, wet-laying or other known operations.
Cellulose paper has traditionally been used as a single ply in which it provides dust filtration and containment, as well as the strength and abrasion resistance required of a vacuum cleaner bag or filter cartridge. This material is rigid enough to enable easy fabrication on standard bag manufacturing or pleating equipment. The paper is predominantly composed of wood pulp, but may have some synthetic fiber reinforcement.
One popular adhesive binder composition for cellulose paper filter products is a poly vinyl acetate. However, other thermoplastic polymers are also commonly used as binders in the manufacture of cellulose paper filters.
The standard cellulose paper filter media typically has a basis weight of about 30–200 g/m2 and commonly about 100 g/m2. This media may have an air permeability in the range of about 20–200 Frazier. These filters do not have the filtration efficiency of higher performance media, such as membrane media. The open structure of cellulose media results in rapid clogging with dust. Moreover, the dust holding capacity is limited by the media thickness. In certain applications, only thin filter media can be used.
A significant development in the area of particle filtration was achieved when expanded PTFE membrane was incorporated as a surface laminate on synthetic depth filter elements. One example is taught in U.S. Pat. No. 5,207,812, directed to a filter cartridge for removing particles of dust from a stream of moving gas or air. Preferred filter media for the cartridge are spun bond or non woven composites containing a layer of porous expanded polytetrafluoroethylene membrane. In household vacuums, filter elements constructed of an ePTFE membrane laminated under heat and pressure to a polyester support are commercially available.
Use of expanded PTFE membranes greatly enhanced the performance of such filter elements because the particles collected on the surface of the expanded PTFE, rather than in the depth of the element, as was occurring in the absence of the expanded PTFE layer. Several significant advantages were observed with these filter elements; first, the filtration efficiency of the elements was high immediately from the outset of the filtration process, and it was not necessary to “build up” a cake of particles to achieve high efficiency; second, the elements lasted longer because particles were not getting into the backing fabric of the element and clogging the element; and third, the cleaning energy needed to clean the dust cake off of the elements was lower because dust cake adhesion to the membrane surface is lower.
The membrane is typically laminated directly to a nonwoven thermoplastic polymer backer under heat and pressure. Known means for adhering the membrane filter media to other support structures include adhering by the use of a separate adhesive. Suitable adhesives may include hot melt polyimides, polyamides, silicones, polyesters, epoxies, polyurethanes, and the like.
Filter media manufactured in accordance with the procedures described above is effective, but relatively expensive. Materials such as meltblown or spun bond polyester are known to be suitable for welding or fusing to ePTFE membranes, but are costly. Such materials may amount to more than 25% of the finished filter media cost. Using a separate adhesive also adds process complexity and cost.
What is needed is a filter media that combines membrane filtration media performance with low cost cellulose paper media without the use of a separate adhesive.